Optics plays a key role in enabling today's information technology revolution by providing a means for high bandwidth communications. Optical transmission bandwidths have been increasing at rates exceeding the famous Moore's Law of the semiconductor electronics industry. Single channel data rates of 10 Gb/s are common, and 40 Gb/s systems are on the horizon. Wavelength division multiplexing (WDM) allows for transmitting multiple (i.e., tens or even hundreds) of lightwave signals at different wavelengths (“wavelength channels”) over the same optical fiber. To date, WDM techniques having data transmission rates exceeding 1 Tb/s have been demonstrated.
To sustain this growth, new technologies are needed both to monitor and to mitigate new transmission impairments attendant with rapidly increasing data rates. In particular, effects related to the polarization state of light (i.e., the orientation of the electric field vector associated with the lightwave signal) have recently emerged as key issues in extending the performance of lightwave systems. The state of polarization (SOP) of light emerging from a length of single-mode optical fiber is usually scrambled, even for input light with a well defined SOP. This polarization scrambling results from very small and essentially random perturbations to the circular symmetry of long fiber optic cables arising both from the manufacturing process and from environmental perturbations.
Polarization effects impact lightwave system performance in several important ways. For example, polarization-dependent loss (PDL) occurs because transmission, reflection, and diffraction of light from optical components is typically a function of polarization. Thus, the PDLs of the individual components of a lightwave system combine to affect system performance, particularly since the SOP may be scrambled between the different components. Lightwave systems must therefore be designed to withstand even worst-case PDL effects. Resulting system impairments from PDL can be mitigated either by reducing the PDLs of individual components (which usually adds cost) or by measuring, characterizing and controlling the SOP within the lightwave system.
Also, in some wavelength-division multiplexed (WDM) communication systems (e.g., undersea lightwave transmission systems), adjacent wavelength channels are orthogonally polarized. This allows neighboring channels to be separated both on the basis of wavelength and polarization, and enables closer channel spacing (in wavelength) than would be possible without the use of orthogonal polarizations. On the other hand, any frequency-dependent polarization scrambling that degrades the orthogonality of adjacent channels leads to polarization cross-talk and impaired system performance. Therefore, the wavelength-dependent polarization behavior of such systems must be measured, characterized and controlled.
Further, polarization scrambling in fiber optic propagation arises from random birefringence in the fiber. Random birefringence in optical fibers may be modeled as a series of randomly oriented wave plates, each of which induces a small differential delay between the polarization components aligned along the fast and slow waveplate axes. The output signal experiences wavelength-dependent polarization scrambling, as well as a wavelength-dependent and input-polarization-dependent delays. These polarization-mode dispersion (PMD) effects lead to random signal fading and increased digital error rates. PMD is already seen as an important adverse effect in today's 10 Gb/s per channel lightwave systems and as a key limiting impairment in industry's quest to move to 40 Gb/s and beyond. The dominant scheme for PMD compensators requires measuring the SOP, manipulating the SOP, and then introducing a controlled polarization-dependent delay.
It is known that a polarized light beam is completely defined by the four Stokes parameters S0, S1, S2 and S3. The parameter So is the total power (e.g., intensity) of the beam. The parameter S1 is the relative power of the vertical versus horizontal polarization component. The parameter S2 is the relative power of the +45° polarization component versus the −45° polarization component. The parameter S3 is the relative power of the right-circular polarization component versus the left-circular polarization component.
In all of the above examples, measuring the SOP (e.g., via determination of the Stokes parameters) is important. However, present day lightwave polarization measurement systems have significant shortcomings for WDM optical communications mainly because they can only measure one wavelength channel at a time.
By way of example, FIG. 1A is a schematic diagram of a prior art polarization measurement system 10. System 10 includes three polarization-independent beam splitters 14 and two mirrors 16, arranged as shown, that break up an input polarized light beam 18 into four separate polarized beams 20A, 20B, 20C and 20D. These four beams are used to separately measure the four Stokes parameters. System 10 includes polarizers 22A, 22B, 22C and 22D placed in the path of beams 20A, 20B, 20C and 20D aligned respectively at 0°, 45°, 90°, and 0°. Associated with and arranged upstream of polarizer 22D is a quarter wave plate 24 so that circularly polarized light (e.g., right-hand circular polarization) can be measured. System 10 includes four photodetectors 30A, 30B, 30C and 30D respectively arranged immediately downstream of polarizers 22A, 22B, 22C and 22D. Photodetectors 30A, 30B, 30C and 30D are each connected to a controller 40.
Measuring the intensity of each of polarized beams 20A, 20B, 20C and 20D with respective detectors 30A, 30B, 30C and 30D allows the four Stokes parameters to be measured, which in turn allows the SOP of input beam 18 to be completely determined (i.e., “characterized”), e.g., by computation in controller 40.
FIG. 1B is a schematic diagram of another prior art polarization measurement system 50 that is more compact than system 10 of FIG. 1A. System 50 includes, in order along an axis A1, a set 56 of adjustable (i.e., movable) waveplates 60, a polarizer 66 with a fixed orientation, and a photodetector 70 coupled to a controller 80. Waveplates 60 are adjusted (e.g., variable-thickness plates are moved relative to one another to vary the overall retardation) to obtain the four polarization orientations needed to calculate the Stokes parameters. The intensity measurements for each setting of adjustable waveplates 60 are made sequentially by detector 70 and stored in controller 80. When the measurements for each setting are completed, the Stokes parameters for polarized light beam 18 are calculated.
While prior art polarization measurement systems such as those described above are suitable for measuring one wavelength channel at a time, they are unsuitable for lightwave transmission systems that employ multiple wavelengths. Because the trend in lightwave communications is to multiplex and transmit optical signals (i.e., lightwaves) having multiple (e.g., tens or hundreds of) wavelengths, there is a need for systems and methods that allow for quickly measuring and characterizing the SOP for each of the different wavelengths (i.e., wavelength channels) in the multi-wavelength beam.